PREFACE. 7 



Such representative models, or constructs, now, in 

 my theory of heredity, are the determinants, which 

 may be conceived as indefinitely fashioned packages 

 of units (biophores) which are set into activity by 

 definite impressions and put a distinctive stamp upon 

 some small part of the organism, on some cell or 

 group of cells, evoking definite phenomena somewhat 

 as a piece of fireworks when lighted produces a bril- 

 liant sun, a shower of sparks, or the glowing characters 

 of a name. 



The ids, also, are such representative models, and 

 may be compared to a definitely ordered but variously 

 compounded aggregate of fireworks, in which the 

 single pieces are so connected as to go off in fixed 

 succession and to produce a definite resultant phenom- 

 enon like a complete inscription surrounded by a hail 

 of fire and glowing spheres. 



Owing to the greater complexity of the phenomena 

 in biology we can never hope to reach the same dis- 

 tinctness in our constructs and models as in physics, 

 and the attempt to derive from them mathematical 

 formulae by the independent development of which 

 research could be continued, would at present be 

 utterly fruitless. In the meantime it seems preferable 

 to have some sort of adequate model to which the 

 imagination can always resort and with which it can 

 easily operate, rather than to have to revert, in con- 

 sidering every special problem of heredity, to the 

 mutual actions of the molecules of living substance 

 and outward agents processes which we know only 

 in their roughest outlines. Or is any one presumptu- 

 ous enough to believe we can infer from our slight 

 knowledge of the chemical and physical constitution 

 of the germs of a trout and a salmon the real cause 



